Conventional pulse width modulation (PWM) electrical drive systems for polyphase AC motors operate in a voltage-controlled mode using a PWM triangulation method. According to such method, an isosceles triangle carrier wave is compared with a fundamental frequency modulating sine wave so that the points of intersection determine the switching instants of the inverter's switching devices. The modulation frequency is usually an integer multiple of the maximum fundamental frequency. Moreover, the carrier-modulation frequency ratio is generally selected to be a multiple of three in order to avoid triple-frequency related currents in the line.
Induction machines operating in the range from zero speed to the motor corner point speed are operating in what is characteristically referred to as the constant torque region; and, above the corner point speed, operation is in a reduced torque, constant power region (power being proportional to the product of motor speed and torque). Commercial drives which use a PWM triangulation method of operation have voltage limitations when operating near the corner point speed. Therefore, low cost PWM drives that use a fixed modulation or chopping frequency are usually limited to operation only in the constant torque region, and maximum speed is slightly less than the corner point speed. As a further limitation, such drives requires substantially constant DC link voltage and hence are not suitable for applications such as are common in the automotive and transportation industries, where the magnitude of DC link voltage can vary over a relatively wide range.
Known AC motor drive systems which operate over a wide speed range use relatively complex control techniques to smoothly transition from PWM mode in the constant torque region to square wave mode in the constant power region. In such drive systems, modulation frequency is decreased as motor speed approaches the corner point speed. Disadvantageously, voltage and current transients typically occur in the speed region near the corner point speed until the transition from PWM mode to square wave mode is complete.
Although vector-controlled drives are available which selectively control chopping frequency during the transition region, these drive systems are even more complex and expensive than those hereinabove described.